Heat generation method in an emission control device

ABSTRACT

A fuel vapor purging method controls fuel vapor purging during stratified operation. Several factors influence vapor purge control, including fuel vapor concentration in the cylinder and temperature of the emission control device. The fuel vapor passes through the cylinder unburned by maintaining the concentration within allowable limits. The unburned fuel vapor reacts exothermically in the emission control device thereby generating heat. To guarantee that the fuel vapor reacts in the first emission control device, the fuel vapor purge is restricted to a certain temperature range. To guarantee that the fuel vapor does not burn in the cylinder, the concentration is kept to a restricted value.

FIELD OF THE INVENTION

The field of the invention relates to fuel vapor purge in directinjection spark ignition engines.

BACKGROUND OF THE INVENTION

In direct injection spark ignition engines, the engine operates at ornear wide-open throttle during stratified air/fuel operation in whichthe combustion chambers contain stratified layers of different air/fuelmixtures. The strata closest to the spark plug contains a stoichiometricmixture or a mixture slightly rich of stoichiometry, and subsequentstrata contain progressively leaner mixtures. The engine may alsooperate in a homogeneous mode of operation with a homogeneous mixture ofair and fuel generated in the combustion chamber by early injection offuel into the combustion chamber during its intake stroke. Homogeneousoperation may be either lean of stoichiometry, at stoichiometry, or richof stoichiometry.

Direct injection engines are also coupled to conventional three-waycatalytic converters to reduce CO, HC, and NOx. When operating atair/fuel mixtures lean of stoichiometry, a NOx trap or catalyst istypically coupled downstream of the three-way catalytic converter tofurther reduce NOx.

Direct injection engines are also coupled to fuel vapor recovery systemsto allow purging of fuel vapors. Conventional systems allow fuel vaporpurging in the stratified mode only when the catalyst temperature ishigh enough to convert the unburned hydrocarbons. In other words, sincethe fuel vapor is inducted with the fresh charge, it forms a homogenousair/fuel mixture. Then, when the fuel is directly injected during thecompression stroke to form a stratified mixture, only the stratifiedfuel burns since the homogenous fuel vapor mixture is too lean.Therefore, unburned fuel vapor may exit the engine cylinder. In somecircumstances, to remove these vapors, the catalytic converter may haveto be above a certain temperature, known to those skilled in the art asthe light-off temperature. Therefore, fuel vapor purging during thestratified mode is restricted until the catalyst has reached thislight-off temperature. Such a method is described in U.S. Pat. No.5,245,975.

The inventor herein has recognized a disadvantage with the aboveapproach. When operating in the stratified mode, less heat is generatedto increase or maintain catalyst or NOx trap temperature. Thus, it takesa long time after a cold start before purging is allowed. Also, withsome catalyst formulations and catalyst configurations, the catalyst maynot maintain this light-off temperature during certain conditions andpurging is only possible in the homogeneous mode, thereby reducing fueleconomy and allowable purge time. In other words, when operating in thestratified mode, catalyst or NOx trap temperature may fall below thelight-off temperature, preventing purge and thereby limiting stratifiedoperation.

SUMMARY OF THE INVENTION

An object of the invention herein is to generate heat in an emissioncontrol device coupled to an engine.

The above object is achieved, and problems of prior approaches overcome,by a method for controlling fuel vapor purge entering an engine, theengine having multiple combustion chambers, the engine coupled to anemission control device, the method comprising the steps of operating ina stratified mode where fuel is injected during a compression stroke ofthe engine; purging fuel vapors into the combustion chambers; andlimiting said purged fuel vapors to be less than a maximum value withsaid maximum value based on cylinder fuel vapor concentration.

By limiting fuel vapor purging based on cylinder fuel vaporconcentration, it is possible to guarantee that the fuel vapors willpass through the combustion chamber unburned. These unburned fuelvapors, along with oxygen from the unburned portion of the chargeinducted, will react exothermically in the emission control device. Thisexothermic reaction will generate heat that will increase the emissioncontrol device to a desired level.

In other words, when purging fuel vapors during stratified operation,the fuel vapors are inducted into the cylinder and may form ahomogeneous mixture that is too lean to combust, even when the engine isoperating in a stratified mode. The point at which the fuel vapors forma combustible mixture is related to the cylinder vapor concentration.Therefore, limiting the fuel vapor purge in this way allows the fuelvapors to form only incombustible mixtures.

An advantage of the present invention is increased fuel vapor purgingduring stratified operation.

Another advantage of the present invention is increased stratifiedoperation and therefore increased fuel economy.

Yet another advantage of the present invention is decreased emissions.

Still another advantage of the present invention is that the addition offuel vapor purge produces minimal, slowly changing, additional torque,allowing smooth transitions into and out of fuel vapor purge operation.

Other objects, features and advantages of the present invention will bereadily appreciated by the reader of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The object and advantages of the invention claimed herein will be morereadily understood by reading an example of an embodiment in which theinvention is used to advantage with reference to the following drawingswherein:

FIG. 1 is a block diagram of an embodiment in which the invention isused to advantage; and

FIGS. 2-10 are high level flowcharts describing a portion of operationof the embodiment shown in FIG. 1.

DESCRIPTION OF AN EMBODIMENT

Direct injection spark ignited internal combustion engine 10, comprisinga plurality of combustion chambers, is controlled by electronic enginecontroller 12. Combustion chamber 30 of engine 10 is shown in FIG. 1including combustion chamber walls 32 with piston 36 positioned thereinand connected to crankshaft 40. In this particular example, piston 30includes a recess or bowl (not shown) to help in forming stratifiedcharges of air and fuel. Combustion chamber 30 is shown communicatingwith intake manifold 44 and exhaust manifold 48 via respective intakevalves 52 a and 52 b (not shown), and exhaust valves 54 a and 54 b (notshown). Fuel injector 66 is shown directly coupled to combustion chamber30 for delivering liquid fuel directly therein in proportion to thepulse width of signal fpw received from controller 12 via conventionalelectronic driver 68. Fuel is delivered to fuel injector 66 by aconventional high pressure fuel system (not shown) including a fueltank, fuel pumps, and a fuel rail.

Intake manifold 44 is shown communicating with throttle body 58 viathrottle plate 62. In this particular example, throttle plate 62 iscoupled to electric motor 94 so that the position of throttle plate 62is controlled by controller 12 via electric motor 94. This configurationis commonly referred to as electronic throttle control (ETC] which isalso utilized during idle speed control. In an alternative embodiment(not shown), which is well known to those skilled in the art, a bypassair passageway is arranged in parallel with throttle plate 62 to controlinducted airflow during idle speed control via a throttle control valvepositioned within the air passageway.

Exhaust gas oxygen sensor 76 is shown coupled to exhaust manifold 48upstream of catalytic converter 70. In this particular example, sensor76 provides signal UEGO to controller 12 which converts signal UEGO intoa relative air/fuel ratio λ. Signal UEGO is used to advantage duringfeedback air/fuel control in a conventional manner to maintain averageair/fuel at a desired air/fuel ratio.

Conventional distributorless ignition system 88 provides ignition sparkto combustion chamber 30 via spark plug 92 in response to spark advancesignal SA from controller 12.

Controller 12 causes combustion chamber 30 to operate in either ahomogeneous air/fuel mode or a stratified air/fuel mode by controllinginjection timing. In the stratified mode, controller 12 activates fuelinjector 66 during the engine compression stroke so that fuel is sprayeddirectly into the bowl of piston 36. Stratified air/fuel layers arethereby formed. The strata closest to the spark plug contains astoichiometric mixture or a mixture slightly rich of stoichiometry, andsubsequent strata contain progressively leaner mixtures. During thehomogeneous mode, controller 12 activates fuel injector 66 during theintake stroke so that a substantially homogeneous air/fuel mixture isformed when ignition power is supplied to spark plug 92 by ignitionsystem 88. Controller 12 controls the amount of fuel delivered by fuelinjector 66 so that the homogeneous air/fuel mixture in chamber 30 canbe selected to be at stoichiometry, a value rich of stoichiometry, or avalue lean of stoichiometry. The stratified air/fuel mixture will alwaysbe at a value lean of stoichiometry, the exact air/fuel being a functionof the amount of fuel delivered to combustion chamber 30. An additionalsplit mode of operation wherein additional fuel is injected during theexhaust stroke while operating in the stratified mode is available. Anadditional split mode of operation wherein additional fuel is injectedduring the intake stroke while operating in the stratified mode is alsoavailable, where a combined homogeneous and split mode is available.

Nitrogen oxide (NOx) absorbent or trap 72 is shown positioned downstreamof catalytic converter 70. NOx trap 72 absorbs NOx when engine 10 isoperating lean of stoichiometry. The absorbed NOx is subsequentlyreacted with HC and catalyzed during a NOx purge cycle when controller12 causes engine 10 to operate in either a rich homogeneous mode or astoichiometric homogeneous mode.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, anelectronic storage medium for executable programs and calibrationvalues, shown as read-only memory chip 106 in this particular example,random access memory 108, keep alive memory 110, and a conventional databus.

Controller 12 is shown receiving various signals from sensors coupled toengine 10, in addition to those signals previously discussed, including:measurement of inducted mass air flow (MAF) from mass air flow sensor100 coupled to throttle body 58; engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling sleeve 114; a profile ignitionpickup signal (PIP) from Hall effect sensor 118 coupled to crankshaft40; throttle position TP from throttle position sensor 120; and absoluteManifold Pressure Signal MAP from sensor 122. Engine speed signal RPM isgenerated by controller 12 from signal PIP in a conventional manner andmanifold pressure signal MAP provides an indication of engine load.

In this particular example, temperature Tcat of catalytic converter 70and temperature Ttrp of NOx trap 72 are inferred from engine operationas disclosed in U.S. Pat. No. 5,414,994, the specification of which isincorporated herein by reference. In an alternate embodiment,temperature Tcat is provided by temperature sensor 124 and temperatureTtrp is provided by temperature sensor 126.

Fuel system 130 is coupled to intake manifold 44 via tube 132. Fuelvapors (not shown) generated in fuel system 130 pass through tube 132and are controlled via purge valve 134. Purge valve 134 receives controlsignal PRG from controller 12.

Generating Heat in an Emission Control Device

Heat generated in an emission control device can be used to advantageto, for example, rapidly increase temperature of the device to obtainfaster light-off during a cold start, control temperature of the deviceto obtain optimum conversion efficiency, purge the device to removesulfur contamination, or many other applications where it isadvantageous to generate heat in the device. Heat is generated in thedevice by providing unburned fuel vapor (HC) from fuel system 130 alongwith excess oxygen from lean combustion. This is done by operating theengine in a stratified mode and introducing fuel vapors during theinduction stroke, thereby forming a homogeneous fuel vapor mixture at alean air/fuel ratio that will not support burning of the fuel vapor.Therefore, the fuel vapors pass through the cylinder unburned and reactexothermically with the excess oxygen since the overall air/fuel ratiois still lean of stoichiometry.

According to the present invention, the device can be any emissioncontrol device such as a three-way catalytic converter or a NOx trap. Inthe embodiment described by FIG. 1, the upstream device is catalyst 70.In an alternative embodiment (not shown), the upstream device may betrap 72.

Limiting Fuel Vapor Purge Based on Emission Control Device Temperature

In order to maximize the heat generated from fuel vapor and excessoxygen exothermically reacting, the emission control device should begreater than a lower threshold but less than a upper threshold. Thelower threshold represents the temperature above which exothermicreactions are supported. Note that this threshold temperature forsupporting exothermic reactions is different than the light-offtemperature, since the light-off temperature is the temperature at whichthe emission control device reaches high efficiency in convertingcertain compounds into alternate compounds, thereby reducing certainregulated emissions.

The upper threshold represents a temperature at which the emissioncontrol device reaches high efficiency and no longer needs additionalheat. Note that the upper threshold may be equal to the light-offtemperature. Also, the upper and lower thresholds are a function ofcatalyst washcoat chemistry.

Calculating Cylinder Fuel Vapor Concentration

To calculate cylinder fuel vapor concentration (ρ_(fc)), fuel vaporconcentration (ρ_(f)) from the fuel vapor system is first calculatedaccording to the following equation:$\rho_{f} = \frac{{\overset{.}{m}}_{p} + {\overset{.}{m}}_{a} - {\lambda \quad S{\overset{.}{m}}_{f}}}{{\overset{.}{m}}_{p}\left( {1 + {\lambda \quad S}} \right)}$

Cylinder fuel vapor concentration (ρ_(fc)) is calculated as:$\rho_{fc} = \frac{{\overset{.}{m}}_{p}\rho_{f}}{\left( {{\overset{.}{m}}_{p} + {\overset{.}{m}}_{a}} \right)}$

where ({dot over (m)}_(p)) is total purge flow rate, ({dot over(m)}_(f)) is actual fuel flow rate from the fuel injectors, ({dot over(m)}_(a)) is fresh charge measured by the air flow meter, (λ) ismeasured relative air/fuel ratio in the exhaust, and S is thestoichiometric air/fuel ratio.

The fuel vapor concentration (ρ_(p)) is thereby calculated based ontotal purge flow rate ({dot over (m)}_(p)) from the fuel vapor system,fresh charge measured by the air flow meter ({dot over (m)}_(a)), therelative air/fuel ratio in the exhaust (λ), the fuel flow rate from thefuel injectors ({dot over (m)}_(f)), and the stoichiometric air/fuelratio S. The cylinder fuel vapor concentration (ρ_(fc)) is then based onfuel vapor concentration (ρ_(f)),total purge flow rate ({dot over(m)}_(p)), and fresh charge measured by the air flow meter ({dot over(m)}_(a)).

In this way, and according to the above equations, the cylinder fuelvapor concentration (ρ_(fc)) is calculated based on availableinformation and can be used to advantage. For example, the calculatedcylinder fuel vapor concentration (ρ_(fc)) can be used to estimateexpected emissions using characteristic relationships between cylindervapor concentration and certain regulated emissions. Further, cylinderfuel vapor concentration (ρ_(fc)) can be used to control many enginecontrol signals such as, for example, ignition timing or injectiontiming during stratified operation. For example, when operating in asplit mode where fuel vapor purge forms the homogenous mixture anddirect fuel injection during forms the stratified mixture, the fuelinjection timing during the compression stroke to form the stratifiedfuel is adjusted based on the cylinder vapor concentration to obtainmaximum torque.

Limiting Fuel Vapor Purge Based on Cylinder Fuel Vapor Concentration

In order to generate heat in the emission control device from theexothermic reaction of fuel vapor and excess oxygen, the fuel vapor mustpass through the cylinder unburned. To guarantee fuel vapor passesunburned, cylinder fuel vapor concentration is limited by a maximumallowable purge concentration (ρ_(fc_(MAX)))

In other words, the calculated cylinder fuel vapor concentration(ρ_(fc)) can be compared to the maximum allowable purge concentration(ρ_(fc_(MAX)))

and used to limit the amount of total purge flow rate ({dot over(m)}_(p)). Allowing the calculated cylinder fuel vapor concentration(ρ_(fc)) to exceed the maximum allowable, or critical, purgeconcentration (ρ_(fc_(MAX)))

may also cause a change in engine torque.

In a preferred embodiment, the maximum allowable, or critical, totalpurge flow ({dot over (m)}_(Pcrit)) is given as:${\overset{.}{m}}_{p_{crit}} = \frac{{\rho_{{fc}_{\max}}{{\overset{.}{m}}_{a}\left( {1 + {\lambda \quad S}} \right)}} - {\overset{.}{m}}_{a} + {\lambda \quad S{\overset{.}{m}}_{f}}}{\rho_{{fc}_{\max}} + {\lambda \quad S\quad \rho_{{fc}_{\max}}} - 1}$

Therefore, to prevent cylinder fuel vapor concentration (ρ_(fc)) fromexceeding the maximum allowable, or critical, purge concentration(ρ_(fc_(MAX))),

purge valve 134 is controlled to limit total purge flow ({dot over(m)}_(p)) to the maximum allowable, or critical, total purge flow ({dotover (m)}_(Pcrit)) In this way, fuel vapor purge will pass through thecylinder unburned, allowing exothermic reaction in an emission controldevice and preventing a change in engine torque.

If cylinder fuel vapor concentration (ρ_(fc)) exceeds the maximumallowable, or critical, purge concentration (ρ_(fc_(MAX))),

then addition engine torque will be produced since some fuel vapors willburn during the engine power stroke. Under these conditions, otherengine control variables are used to keep engine torque constant suchas, for example, fresh air charge, ignition timing, exhaust gasrecirculation, fuel injection amount, fuel injection timing, and/orvariable cam timing. For example, rapid engine torque control can beachieved by using ignition timing or fuel injection amount. Inparticular, if using fuel injection amount, the amount of fuel injectionamount adjustment can be determined by estimating an amount of fuelvapor purge that is burned, and this amount subtracted from the normalfuel injection amount. The following equation shows the calculation,where fia_c is the adjusted fuel injection amount, fia is the normalfuel injection amount (when fuel vapor purge does not exceed the maximumallowable concentration), and fvp_b is the amount of fuel vapor purgeburned (determined using predetermined maps relating how far the actualpurge concentration exceeds the maximum allowable concentration):

fia _(—) c=fia−fvp _(—) b

If one of the other engine control variables is used, predeterminedcharacteristic maps relating engine torque to the control variable areused to maintain constant engine torque.

Discontinuing Stratified Fuel Vapor Purge

In some cases, limiting fuel vapor purge based on a maximum allowablefuel vapor cylinder concentration causes insufficient purging to keep avapor system from becoming saturated. Thus, is some cases it isnecessary to discontinue stratified fuel vapor purging when the actuallimited fuel vapor purge is less than a desired value. In this case,either a split mode or homogenous mode is selected to allow maximum fuelvapor purge, where the fuel vapor purge is burned in the cylinder.

For example, when actual fuel vapor purge (limited or clipped in anyway, or determined through temperature control as described laterherein), is less than a minimum required value, operation where fuelvapor purge passes through the cylinder is discontinued. The engineswitches to either a purely homogenous mode or a split mode where purgeis not limited or clipped and increase fuel vapor purge is possible. Inthese modes, the fuel vapor purge is burned in the cylinder and does notcreate an exothermic reaction in the catalyst.

Using Cylinder Fuel Vapor to Control Emission Device Temperature

As stated previously herein, heat generated by purging fuel vaporsduring stratified operation can be used to control temperature of theemission control device when the exothermic reaction is taking place.For example, the amount of fuel vapor purge, which is proportional toexothermic heat creating in the emission control device, can becontrolled to maintain a desired emission control device temperature.When the emission control device temperature falls below a desired valueand the engine is operating stratified, fuel vapor from fuel system 130can be introduced into the cylinder and pass to the emission controldevice unburned. The fuel vapor then reacts exothermically in theemission control device, thereby generating heat and increasingtemperature of the emission control device. As the temperature of theemission control device approaches the desired value, less fuel vaporpurge is introduced, thereby causing temperature of the emission controldevice to converge to the desired level.

Opportunistic Fuel Vapor Purge Based on Emission Control DeviceTemperature.

When the emission control device temperature is in a lower portion of anacceptable range, fuel vapor purge can be allowed to take advantage of afuel vapor purge opportunity. In other words, if it is necessary topurge fuel vapor because fuel vapor storage has become saturated, fuelvapor purge can be enabled as long as the emission control device cantolerate increased temperature. In this way, increased fuel vapor purgetime is obtained while keeping the emission control device withinacceptable temperature limits.

Using Cylinder Fuel Vapor Concentration to Control A Second EmissionDevice Temperature Downstream of A First Emission Control Device

In addition, heat generated by purging fuel vapors during stratifiedoperation can also be used to control temperature of a second emissioncontrol device downstream of the emission control device where theexothermic reaction is taking place. In other words, the heat generatedin the upstream device also affects the downstream emission controldevice temperature and thus can be used to control the temperature ofthe downstream emission control device.

While controlling downstream device temperature, limits on controlauthority for limiting exothermic reaction in the upstream device arenecessary to prevent upstream device temperature from becoming greaterthan a maximum allowable device temperature. In this configuration, fuelvapor purge is controlled according to a difference between a desiredminimum temperature for the downstream emission control device andactual temperature of the downstream emission control device.

Also, combined temperature control is possible where fuel vapor purge iscontrolled to maintain both temperature of the upstream and downstreamemission control device. In this configuration, fuel vapor is controlledaccording to both the difference between a desired minimum temperaturefor the downstream emission control device and actual temperature of thedownstream emission control device and the difference between a desiredminimum temperature for the upstream emission control device and actualtemperature of the upstream emission control device.

Discontinuing Stratified Fuel Vapor Purge Based on Total Fuel Used toProduce the Current Desired Torque.

By injecting fuel vapors into the engine during stratified operation andallowing them to pass through unburned to maintain emission controldevice temperature, stratified operation is extended, thus giving a fueleconomy benefit. However, if the total amount of fuel used in thisoperation mode, including fuel vapor and fuel injected to the formstrata that are burning, is greater than the amount of fuel that wouldbe used during homogeneous operation to produce equal engine torque,then fuel economy is degraded by continuing stratified operation. Statedanother way, when the stratified mode no longer provides a fuel savingsbecause of the additional fuel vapor injected to maintain temperature ofthe emission control device, the engine air/fuel operating mode isswitched to the homogeneous mode thereby maximizing fuel economy. Inaddition, fuel vapor purge during the homogenous mode is still allowedsince the additional fuel vapor will burn and produce engine torque.

To determine the mode, the following criteria is used:${\rho_{f}{\overset{.}{m}}_{p}} + \overset{.}{m_{j} > \overset{.}{m_{f_{stoich}}}}$

where, $\left( {\overset{.}{m}}_{f_{{stoich}\quad}} \right)$

is the amount of fuel necessary during homogenous operation to produceequal engine torque to that produced by actual fuel flow rate from thefuel injectors {dot over (m)}_(f) during stratified mode. As the engineswitches to the homogeneous mode, the new fuel flow rate from the fuelinjectors $\left( {\overset{.}{m}}_{f_{h}} \right)$

is found as:${\overset{.}{m}}_{f_{h}} = {{\overset{.}{m}}_{f_{stoich}} - {\rho_{f}{\overset{.}{m}}_{p}}}$

when fuel vapor purge is continued in the homogenous mode.

Referring now to FIG. 2, a routine for controlling temperature (Tcat) ofcatalyst 70 is described. First, in step 210 a determination is made asto whether engine 10 is operating in a stratified mode where fuel isinjected directly into cylinder 30 during a compression stroke. When theanswer to step 210 is YES, a determination is made as to whethertemperature Tcat of catalyst 70 is between threshold values T1 and T2.Threshold value T2 represents an upper level above which catalyst 70 isoperating a peak efficiency and thus control of temperature (Tcat) isnot required. Temperature threshold T1 represents a lower level belowwhich catalyst 70 cannot support an exothermic reaction between unburnedfuel vapors and excess oxygen. When catalyst temperature Tcat is withinthis range, the routine continues to step 214, where the fuel vaporconcentration (ρ_(fc)) in the cylinder is calculated, as will bedescribed later herein with particular reference to FIG. 3.

Continuing with FIG. 2, a temperature error (e) is calculated in step216 based on a desired temperature (Td) of catalyst 70 and the actualtemperature (Tcat) of catalyst 70. Then, in step 218, an initial controlvalue (PRG_temp) is calculated based on a function (f) of error (e). Ina preferred embodiment, function (f) represents a proportional,integral, and derivative controller, known to those skilled in the artas a PID controller. Then, in step 220, the initial control value(PRG_temp) is clipped based on the calculated fuel vapor concentration(ρ_(fc)) in the cylinder, as described later herein with particularreference to FIG. 4. This operation prevents the temperature controllerfrom adding too much fuel vapor from fuel system 130 and creating acombustible fuel vapor concentration in the cylinder. Finally, in step222, control signal PRG is calculated based on the clipped value(PRG_c).

Referring now to FIG. 3, a routine for calculating the concentration offuel vapors from fuel system 130 (ρ_(f)) and cylinder fuel vaporconcentration (ρ_(fc)) is described. First, in step 310, theconcentration of fuel vapors from fuel system 130 is calculated as:$\rho_{f} = \frac{{\overset{.}{m}}_{p} + {\overset{.}{m}}_{a} - {\lambda \quad S{\overset{.}{m}}_{f}}}{{\overset{.}{m}}_{p}\left( {1 + {\lambda \quad S}} \right)}$

Then, in step 312, the cylinder fuel vapor concentration (ρ_(fc)) iscalculated as:$\rho_{fc} = \frac{{\overset{.}{m}}_{p}\rho_{f}}{\left( {{\overset{.}{m}}_{p} + {\overset{.}{m}}_{a}} \right)}$

where {dot over (m)}_(p) is the total purge flow rate, {dot over(m)}_(f) is the fuel flow rate from the fuel injectors, {dot over(m)}_(a) is fresh charge measured by the air flow meter, λ is themeasured relative air/fuel ratio from the UEGO sensor, and S is thestoichiometric air/fuel ratio. Total purge flow rate, {dot over (m)}_(p)is determined based on the previous control signal PRG where:

{dot over (m)} _(p) =g(PRG)

where (g) is a predetermine function relating command signal PRG tototal flow rate, {dot over (m)}_(p). Thus, it is possible to estimatethe actual fuel vapor concentration from fuel system 130 and cylinderfuel vapor concentration from available measurements.

Referring now to FIG. 4, in step 410 a determination is made as towhether purge concentration is greater than the maximum allowable purgeconcentration (ρ_(fc) _(MAX) ) to guarantee that the fuel vapors passthe cylinder unburned and react exothermically in catalyst 70. Also, asafety factor, SF is also included. When the answer to step 410 is NO,the clipped control value (PRG_c) is set to initial control value(PRG_temp) in step 412. Thus, when the concentration is less than thatwhich will burn in the cylinder, the temperature controller according toFIG. 2 is allowed full authority to change fuel vapor flow through valve134. Continuing with FIG. 4, when the answer to step 410 is YES, acritical purge flow ({dot over (m)}_(Pcrit)) is calculated according tothe following equation:${\overset{.}{m}}_{p_{crit}} = \frac{{\rho_{{fc}_{\max}}{{\overset{.}{m}}_{a}\left( {1 + {\lambda \quad S}} \right)}} - {\overset{.}{m}}_{a} + {\lambda \quad S{\overset{.}{m}}_{f}}}{\rho_{{fc}_{\max}} + {\lambda \quad S\quad \rho_{{fc}_{\max}}} - 1}$

This value represents the maximum allowable total purge flow that givesan unburnable fuel vapor concentration in the cylinder. Next, in step416, the maximum allowable command signal (PRG_MAX) is calculated basedon the critical purge flow ({dot over (m)}_(Pcrit)) and the inverse offunction (g). Finally, in step 418, the clipped value (PRG_c) is setequal to the maximum allowable command signal (PRG_MAX). This preventsthe temperature control described by FIG. 2 from allowing fuel vaporpurge concentration to enter the cylinder that will burn and producetorque. Thus, the system is able to control catalyst temperature whileremaining in a stratified mode and keeping combustion torque constant.

Referring now to FIG. 5, an alternative embodiment is described wherepurge vapors are controlled during stratified operation to maintaintemperature (Ttrp) of a NOx trap 72 downstream of catalyst 70. First, instep 510 a determination is made as to whether engine 10 is operating ina stratified mode where fuel is injected directly into cylinder 30during a compression stroke. When the answer to step 510 is YES, adetermination is made in step 512 as to whether temperature Tcat ofcatalyst 70 is between threshold values T1 and T2. Threshold value T2represents an upper level above which catalyst 70 is operating a peakefficiency. Temperature threshold T1 represents a lower level belowwhich catalyst 70 cannot support an exothermic reaction between unburnedfuel vapors and excess oxygen. In an alternative embodiment, thresholdvalues T1 and T2 can represent threshold temperature of NOx trap 72. Inthis alternative embodiment, temperature of catalyst 70 Tcat may be toolow to support exothermic reaction due to, for example, washoutchemistry. However, NOx trap 72 may be within a temperature range tosupport exothermic reaction because of it different washcoatformulation. Therefore, in this alternative embodiment the temperaturerange for allowing purge during stratified operation can is based ontemperature of both Tcat and Ttrp.

When catalyst temperature Tcat is within this range, or in thealternative embodiment, when trap temperature Ttrp is within the range,the routine continues to step 513, where a determination is made as towhether trap temperature Ttrp is less than control threshold temperatureTt1. If the answer to step 512 is YES, the routine continues to step 513where the fuel vapor concentration (ρ_(fc)) in the cylinder iscalculated as described previously herein with particular reference toFIG. 3.

Continuing with FIG. 5, a temperature error (e) is calculated in step516 based on a desired temperature (Td) of NOx trap 72 and the actualtemperature (Ttrp) of NOx trap 72. Then, in step 518, an initial controlvalue (PRG_temp) is calculated based on a function (f) of error (e). Ina preferred embodiment, function (f) represents a proportional,integral, and derivative controller, known to those skilled in the artas a PID controller. Then, in step 520, the initial control value(PRG_temp) is clipped based on the calculated fuel vapor concentration(ρ_(fc)) in the cylinder, as described previously herein with particularreference to FIG. 4. This operation prevents the temperature controllerfrom adding too much fuel vapor from fuel system 130 and creating acombustible fuel vapor concentration in the cylinder. Finally, in step522, clipped control signal (PRG_c) is limited based on temperature(Tcat) of catalyst 70, as described later herein with particularreference to FIG. 6 or FIG. 8.

Referring now to FIG. 6, a routine for limiting clipped control signal(PRG_c) is described so that temperature (Ttrp) of NOx trap 72 can becontrolled to the desired level while preventing temperature (Tcat) frombecoming too high. First in step 610, a determination is made as towhether temperature (Tcat) of catalyst 70 is less than a maximumallowable temperature (TuMAX). When the answer to step 610 is YES,control signal PRG is calculated based on the clipped value (PRG_c).Otherwise, control signal PRG is calculated based on the clipped value(PRG_c) and a function (f2) of maximum allowable temperature (TuMAX) andtemperature (Tcat) of catalyst 70. In a preferred embodiment, function(f2) represents a proportional, integral, and derivative controller,known to those skilled in the art as a PID controller.

Referring now to FIG. 7, another alternative embodiment is describedwhere purge vapors are controlled during stratified operation tomaintain both temperature (Ttrp) of a NOx trap 72 downstream andtemperature (Tcat) of catalyst 70. First, in step 710 a determination ismade as to whether engine 10 is operating in a stratified mode wherefuel is injected directly into cylinder 30 during a compression stroke.When the answer to step 710 is YES, the routine continues to step 514,where the fuel vapor concentration (ρ_(fc)) in the cylinder iscalculated, as described previously herein with particular reference toFIG. 3. Then, in step 712, a determination is made as to whethertemperature Tcat of catalyst 70 is between threshold values T1 and T2.Threshold value T2 represents an upper level above which catalyst 70 isoperating a peak efficiency. Temperature threshold T1 represents a lowerlevel below which catalyst 70 cannot support an exothermic reactionbetween unburned fuel vapors and excess oxygen. When the answer to step712 is YES, a catalyst temperature error (e) is calculated in step 714based on a desired catalyst temperature (Tdcat) of catalyst 70 and theactual temperature (Tcat) of catalyst 70.

Continuing with FIG. 7, in step 716 a determination is made as towhether temperature Ttrp of NOx trap 72 is between threshold values T1′and T2′. Threshold value T2′ represents an upper level above which NOxtrap 72 is operating a peak efficiency. Temperature threshold T1′represents a lower level below which NOx trap 72 cannot support anexothermic reaction between unburned fuel vapors and excess oxygen. Whenthe answer to step 716 is YES, a trap temperature error (e′) iscalculated in step 718 based on a desired trap temperature (Tdtrp) ofNOx trap 72 and the actual temperature (Ttrp) of NOx trap 72. Otherwise,trap temperature error (e′) is set to zero in step 720. Then, in step722, from either step 718 or 720, a total error (etot) is calculatedfrom both trap temperature error (e′) and catalyst temperature error(e). Then, in step 724 an initial control value (PRG_temp) is calculatedbased on a function (f) of total error (etot). In a preferredembodiment, function (f) represents a proportional, integral, andderivative controller, known to those skilled in the art as a PIDcontroller. Then, in step 726, the initial control value (PRG_temp) isclipped based on the calculated fuel vapor concentration (ρ_(fc)) in thecylinder, as described previously herein with particular reference toFIG. 4. This operation prevents the temperature controller from addingtoo much fuel vapor from fuel system 130 and creating a combustible fuelvapor concentration in the cylinder. Finally, in step 728, clippedcontrol signal (PRG_c) is limited based on temperature (Tcat) ofcatalyst 70, as described previously herein with particular reference toFIG. 6.

If the answer to step 712 is NO, then a determination is made in step740 as to whether temperature Ttrp of NOx trap 72 is between thresholdvalues T1′ and T2′. When the answer to step 740 is YES, trap temperatureerror (e′) is calculated in step 742 based on a desired trap temperature(Tdtrp) of NOx trap 72 and the actual temperature (Ttrp) of NOx trap 72.Then, in step 744, catalyst temperature error (e) is set to zero.Otherwise, trap temperature error (e′) is set to zero and catalysttemperature error (e) is set to zero in step 746.

Accordingly, both emission control device temperatures are maintainedabove a value representing peak catalyst efficiency using unburned fuelvapor purge.

Referring now to FIG. 8, a routine for limiting clipped control signal(PRG_c) is described so that temperature (Ttrp) of NOx trap 72 can becontrolled to the desired level while preventing temperature (Tcat) frombecoming too high or stratified operation is discontinued if purge flowis too low. First, in step 810, a determination is made as to whethertemperature (Tcat) of catalyst 70 is less than a maximum allowabletemperature (TuMAX). When the answer to step 810 is YES, intermediatecontrol signal PRG_S is calculated based on the clipped value (PRG_c).Otherwise, intermediate control signal PRG_S is calculated in step 814based on the clipped value (PRG_c) and a function (f2) of maximumallowable temperature (TuMAX) and temperature (Tcat) of catalyst 70. Ina preferred embodiment, function (f2) represents a proportional,integral, and derivative controller, known to those skilled in the artas a PID controller.

Then, in step 820, a determination is made as to whether intermediatecontrol signal PRG_S is greater than required fuel vapor purge REQ_PRG.Required fuel vapor purge REQ_PRG is determined so that fuel system 130does not become oversaturated with fuel vapors. When the answer to step820 is NO, then additional fuel vapor purge is required. Since fuelvapor purge cannot be increased in the current mode without increasingengine torque or emission control device temperature, engine 10 isswitched to homogenous operation in step 822. In an alternativeembodiment, engine 10 is switched a split mode where both a homogeneousmixture and stratified mixture, both of which burn in the enginecylinders, are formed.

Referring now to FIG. 9, a routine for controlling temperature (Tcat) ofcatalyst 70 is described. First, in step 910, a determination is made asto whether engine 10 is operating in a stratified mode where fuel isinjected directly into cylinder 30 during a compression stroke. When theanswer to step 910 is YES, a determination is made as to whethertemperature Tcat of catalyst 70 is between threshold values T1 and T2.Threshold value T2 represents an upper level above which catalyst 70 isoperating a peak efficiency and thus control of temperature (Tcat) isnot required. Temperature threshold T1 represents a lower level belowwhich catalyst 70 cannot support an exothermic reaction between unburnedfuel vapors and excess oxygen. When catalyst temperature Tcat is withinthis range, the routine continues to step 914, where the fuel vaporconcentration (ρ_(fc)) in the cylinder is calculated, as will bedescribed later herein with particular reference to FIG. 3.

Continuing with FIG. 9, a temperature error (e) is calculated in step916 based on a desired temperature (Td) of catalyst 70 and the actualtemperature (Tcat) of catalyst 70. Then, in step 918, an initial controlvalue (PRG_temp) is calculated based on a function (f) of error (e). Ina preferred embodiment, function (f) represents a proportional,integral, and derivative controller, known to those skilled in the artas a PID controller. Then, in step 920, the initial control value(PRG_temp) is clipped based on the calculated fuel vapor concentration(ρ_(fc)) in the cylinder, as described previously herein with particularreference to FIG. 4. This operation prevents the temperature controllerfrom adding too much fuel vapor from fuel system 130 and creating acombustible fuel vapor concentration in the cylinder. Then, in step 922,a determination is made as to whether clipped value (PRG_c) is less thana required purge value to prevent saturation of fuel system 130. If theanswer is YES, then in step 924, stratified operation is discontinues.Thus, engine 10 is operated in homogenous mode and purging is allowed atthe required rate. Otherwise, in step 930, control signal PRG iscalculated based on the clipped value (PRG_c).

Referring now to FIG. 10, a routine for enabling fuel vapor purging isdescribed. First, in step 1010, a determination is made as to whetherengine 10 is operating in a stratified mode where fuel is injecteddirectly into cylinder 30 during a compression stroke. When the answerto step 1010 is YES, a determination is made in step 1012 as to whethertemperature Tcat of catalyst 70 is between threshold values T11 and T22.Threshold value T22 represents an upper level above which efficiency ofcatalyst 70 degrades. Temperature threshold T11 represents a lower levelbelow which catalyst 70 cannot support an exothermic reaction betweenunburned fuel vapors and excess oxygen.

Continuing with FIG. 10, when the answer to step 1012 is YES, adetermination is made in step 1014 as to whether catalyst temperatureTcat is in a lower portion of an allowable purge temperature rangedefined by T11 and T22, where parameter X represents a predeterminedpercentage of the allowable purge temperature range. Parameter X isbased on engine operating conditions such as, for example, engine freshair flow. When the answer to step 1010 is NO or step 1014 is YES, fuelvapor purging is enabled in step 1016. Otherwise, in step 1018, fuelvapor purging is disabled. This concludes a description of an example inwhich the invention is used to advantage. Those skilled in the art willrecognize that many modifications may be practiced without departingfrom the spirit and scope of the invention. For example, the inventioncan be carried out when operating in a purely stratified mode or when ina split mode, where both a stratified and homogenous mixture are formed.In addition, various emission control devices can be substituted foreither catalyst 70 or trap 72. For example, a second trap could be usedin place of catalyst 70. In this example, the routines using temperatureof catalyst 70 would now use temperature of the second trap.Accordingly, it is intended that the invention be limited only by thefollowing claims.

I claim:
 1. A method for controlling fuel vapor purge entering anengine, the engine having multiple combustion chambers, the enginecoupled to an emission control device, the method comprising the stepsof: operating in a stratified mode where fuel is injected during acompression stroke of the engine; purging fuel vapors into thecombustion chambers operating in said stratified mode; and limiting saidpurged fuel vapors to be less than a maximum value, with said maximumvalue based on cylinder fuel vapor concentration.
 2. The method recitedin claim 1 wherein said purging step further comprises the step ofpurging fuel vapors into the combustion chamber so that an emissioncontrol device temperature approaches a desired emission control devicetemperature.
 3. The method recited in claim 1 wherein said purging stepfurther comprises the step of purging fuel vapors into the combustionchamber when an emission control device temperature is less than apredetermined value.
 4. The method recited in claim 1 wherein saidcylinder fuel vapor concentration is based on a fuel injection amount,an engine airflow, a total purge flow rate, and an exhaust air/fuelratio.
 5. The method recited in claim 1 wherein said purging stepfurther comprises purging fuel vapors into the combustion chambers thatwill pass through the chambers unburned, thereby providing unburnedhydrocarbons and excess oxygen to react exothermically and generate heatin the emission control device.
 6. The method recited in claim 1 furthercomprising the step of adjusting determining a desired total purge flowbased on said maximum value and a difference between an emission controldevice temperature and a desired emission control device temperature. 7.The method recited in claim 6 further comprising the step of adjusting apurge flow control valve based on said desired total purge flow.
 8. Acontrol method for an engine having multiple combustion chambers coupledto an emission control device, the method comprising the steps of:operating in a stratified mode where fuel is injected during acompression stroke of the engine; providing an indication of temperatureof the emission control device; determining a fuel vapor concentrationin the combustion chambers operating in said stratified mode; andcontrolling fuel vapors inducted into the combustion chambers based onsaid temperature and said fuel vapor concentration.
 9. The methodrecited in claim 8 further comprising the step of adjusting a fuel vaporcontrol valve to control an amount of fuel vapors being purged.
 10. Themethod recited in claim 9 further comprising the step of adjusting saidfuel vapor control based on said temperature.
 11. The method recited inclaim 10 further comprising the step of adjusting said fuel vaporcontrol so that said temperature approaches a desired temperature. 12.The method recited in claim 8 wherein said controlling step furthercomprises purging fuel vapors into the combustion chambers when saidtemperature is below said predetermined threshold and above apreselected threshold.
 13. The method recited in claim 9 based on apredetermined relationship between a total purge flow and a position ofsaid fuel vapor control valve.
 14. The method recited in claim 8 whereinsaid fuel vapors inducted into the combustion chambers pass through thechambers unburned, thereby providing unburned hydrocarbons and excessoxygen to react exothermically and generate heat in the emission controldevice.
 15. An article of manufacture comprising: a computer storagemedium having a computer program encoded therein for controlling anengine having multiple combustion chambers coupled to an emissioncontrol device, said computer storage medium comprising: code foroperating in a stratified mode where fuel is injected during acompression stroke of the engine; code for providing an indication oftemperature of the emission control device; code for determining a fuelvapor concentration in the combustion chambers operating in a stratifiedmode; and code for controlling fuel vapors inducted into the combustionchambers based on said temperature and said fuel vapor concentration.16. The article recited in claim 15 further comprising code foradjusting a fuel vapor control valve to control an amount of fuel vaporsbeing purged.
 17. The article recited in claim 16 further comprisingcode for adjusting said fuel vapor control based on said temperature.18. The article recited in claim 17 further comprising code foradjusting said fuel vapor control so that said temperature approaches adesired temperature.
 19. The article recited in claim 18 wherein saidcode for controlling fuel vapors inducted into the combustion chambersfurther comprises code for purging fuel vapors into the combustionchambers when said temperature is below said predetermined threshold andabove a preselected threshold.
 20. The article recited in claim 18wherein said fuel vapors inducted into the combustion chambers passthrough the chambers unburned, thereby providing unburned hydrocarbonsand excess oxygen to react exothermically and generate heat in theemission control device.
 21. The article recited in claim 18 whereinsaid emission control device is a NOx trap.